To run RFAssistant you will need to install the Java
Runtime Environment (JRE), version 1.5 or higher. If you are
not interested
in developing Java code, select the "JRE" for your computer and
operating
system. (The "SDK" is the system development kit, which is what
you
need if you want to write programs in Java.) This will install
the
Java Web Start program, which will launch the RFAssistant code.

To run RFAssistant, click on the link below. Be aware,
however, that Java Web Start will give you a security warning that the
code is signed by "William R. Frensley," but that it cannot verify this
and it will recommend that you do not run this program. What this
means is that I have not paid a few hundred dollars per year for a
verified
security certificate. Note that the code is being served from the
University of Texas at Dallas web site, and that I am a Professor at
UT-Dallas
and make your own judgement about the safety of this code.

When RFAssistant launches, you will see the a screen like the
following
figure, but the graph will be blank. The first thing that must be
done is to read in an S-parameter file. If you don't have such a
file downloaded on your computer already, copy the following URL,
select
"Open URL" from the "File" menu, and paste the text into the dialog box
that asks you to enter the URL.
http://www.utdallas.edu/~frensley/courses/EE4368/FHX13X_FHX14X.s2p
Now you can select an S-parameter to plot from the choice box at the
top of the left-hand panel. You should see a display like
the
following:

Selecting a frequency (from the set for which data was available)
causes
the S-parameter at that frequency to be marked with a black dot.

Clicking on the "Stability" tab at the bottom of the frame takes you
to a plot of various stability-related factors as a function of
frequency:

Delta is the determinant of the S-matrix, K is the Rowlett stability
factor, and U is the unilateral figure of merit. Mu is the
stability
factor proposed by Edwards and Sinsky in IEEE Transactions on Mircowave
Theory and Techniques, Vol. 49, pp. 2303-11 [1992]. In general,
the
transistor will be unconditionally stable if K or mu is greater than 1.

Clicking on the "Gain" tab produces a display of the maximum gain as
a function of frequency:

This is probably the most informative display in RFAssistant.
It lets you visualize and understand the interaction between the input
and output conditions in a linear two-port network (typically a
transistor).
Notice the circuit diagram at the bottom of the display. It is the
guide
to the meanings of the quantities displayed on the Smith charts.
The chart to the left shows quantities on the input side.
Clicking
and dragging on this chart changes the value of Gamma S, indicated by
the
"+" mark. When you change this, you will notice that the "X" mark
on the right-hand display also moves. This shows Gamma
OUT*.
A "simultaneous conjugate match" is achieved when we can get Gamma S =
GammaIN* and GammaL = GammaOUT* at the same time. However to do
this,
the transistor has to be unconditionally stable. Our example
transistor
is stable above about 18 GHz. (You will need to change the
frequency
on the "S Parameters" tab.) At a frequency of 20 GHz (as
shown
in the figure) you can alternately move GammaS to chase after GammaIN*
and move GammaL to chase after GammaOUT*. When they get
close
enough, the "Iterate" button on the left-hand panel becomes active, and
you can simply press it a number of times to automate this process.

The "Input" tab produces a display which looks like this:

This is a plot in the GammaS plane with more information
displayable.
In particular, several sets of "design circles" can be
displayed.
These are controlled in the left-hand control panel, in the boxes
labeled
"Gs Circles," "NF Circles," and "Ga Circles." In each case the
plotting
of the set is controlled by a check box, and the contour interval is
selectable
from a choice box.

"Gs" is the "source gain," a concept that is meaningful within
the
unilateral
approximation. The Gs circles are plotted in blue and the maximum
value is located at S11* which is plotted as a blue dot.

"NF" is the amplifier noise figure. The plot is activated
if
noise
data are present in the S-parameter file. The noise figure
contours
are plotted in green.

"Ga" is the available gain. The Ga circles are plotted in
magenta
and reach a maximum at the simultaneous conjugate input match, when
this
exists.

The "Output" tab functions similarly to the Input tab.

The "GL" circles represent the "load gain" within the unilateral
approximation.
They are plotted in blue and reach the maximum value at S22*.

The "Gp" circles represent the power gain. They are plotted
in
green
and reach a maximum at the simultaneous conjugate output match, when
this
exists.

Conditionally Stable Cases

When the transistor is not unconditionally stable, the situation is
somewhat
more complicated. In these cases the displays will appear
somewhat
differently.